1. Field of the Invention
The present invention relates to a polypropylene-laminated steel sheet which is excellent in resistance to peeling, resistance to impact-whitening, and resistance to corrosion of worked portions, and to a process for producing the laminated steel sheet. More specifically, the present invention relates to a polypropylene-laminated steel sheet suitable for containers such as 18-liter cans and aerosol cans, and a process for producing the laminated steel sheet.
2. Description of the Related Art
In a polypropylene-laminated metal sheet, the resistance to peeling of a polypropylene film from a metal sheet such as a steel sheet is improved by use of a polypropylene modified by grafting with an unsaturated carboxylic acid as is disclosed in the prior art publications, e.g., Japanese Patent Publication No. Sho 49-4822, and Japanese Patent Application Laid-Open Nos. Sho 54-90378 and Sho 57-197156. The polypropylene-laminated steel sheet produced by the processes disclosed in the prior art publications involve the problem that the laminated resin layer becomes whitened by impact shock at pressing or other working.
The whitening of the polypropylene laminated layer caused upon impact shock such as during press workings is considered to be attributed to microcracks occurring in the polypropylene layer on receiving impact shock and resultant light scattering at the microcrack portions, or to be attributed to crystallization of the polypropylene resin by stress concentration on application of impact shock and resultant light scattering at the interface between crystalline portions and non-crystalline portions. Once the whitening occurs not only the external appearance of the laminated steel sheet is impaired but also corrosion is initiated from the site of the microcracks, thus lowering the corrosion resistance thereof.
Generally, the polypropylene-laminated steel sheet has been considered to have high corrosion resistance. Therefore, it has been believed that a stress-crack-inducing substance can be packed and stored in a container made of a polypropylene-laminated steel sheet without any problem. However, it has been found that rust is formed around the press-worked portion of a can when a surfactant-containing solution which tends to cause stress-cracking is packed and stored in the can, e.g., a 18-liter can, made of a polypropylene-laminated steel sheet according to the method of the above-mentioned Japanese Patent Laid-Open Publications. This means that the above laminated steel sheet is inferior in corrosion resistance.